Tuesday, June 14, 2011

Aircraft Design 101: What is Maximum Take-Off Weight?

Aircraft Design 101: What is Maximum Take-Off Weight?: "Emirates Boeing 777-300ER A6-ECZ

As you may have noticed, I am not an aerospace engineer. In fact, I'm not an engineer of any discipline. Though over the last several years, I've sought to learn more about aircraft performance, payload capability and design to better understand the industry I cover and I was keen to share what I had learned. Many of you are aircraft designers, so your job is to check my work and recommend where this can be clearer. For those of you are like me - learning - I hope this will become an accessible reference. So, today I give you what may be the first installment of what may become a periodic series: Aircraft Design 101 (as told by a non-engineer). Physics for Poets, if you will.
Every aircraft has a Maximum Take-off Weight (MTOW), which is dictated by the structural capacity of the aircraft. Though within the aircraft's maximum allowed weight are several different elements, each one contributing to the overall performance of the aircraft. If we think of MTOW a glass which cannot be overfilled, inside sits layers of an alphabet soup of additional weights that determine how much an aircraft can carry and how far it can be carried.

Let's take a large jetliner for example and load it to its maximum takeoff weight the moment it begins its takeoff roll. At this particular moment the total weight, or gross weight, of the aircraft is the sum of the aircraft, the amount of fuel it's carrying and what it's carrying. This is also known as the Take-Off Weight (TOW) That is of course an over simplification, but these are the three key ingredients to understanding how much an aircraft weights.

Before any items are installed that make the aircraft usable as a commercial transport, the aircraft itself its made up of the airframe, furnishings, the systems and its propulsion. The sum of these three items are the Manufacturers Empty Weight (MEW). The MEW also includes, for example, hydraulic fluid, which is found in a 'closed' system aboard the aircraft and not consumed.

As it readies for revenue service, many items are added to the aircraft for it to be missionized. For example, the seats, emergency equipment and other consumable fluids such as engine oil, toilet chemicals and fluids, as well as the fuel that can't reach the pickups in the tank, also known as unusable fuel. Naturally, you're not going anywhere without the flight and cabin crew and their baggage. All together, you add the weight of these items to MEW to get the Operational Empty Weight (OEW or OWE) of the aircraft.

Aircraft-Weights.jpgWhile it's parked at the gate and you're watching your ride from the terminal windows, the aircraft is loaded with fresh catering and potable water, your pre-flight newspaper, any pantry equipment and extra crew. Add these weights to the OWE and you have the aircraft's Dry Operating Weight (DOW).

Once everyone is boarded comfortably ready to fly along with their baggage in the overhead bins and in the cargo hold, that weight is added with the pallets with revenue cargo that are flying along with you to your destination. All these items are called the Traffic Load (TL). The TL is then added to the DOW to yield the Zero Fuel Weight (ZFW), before an ounce of usable fuel has been put into the tanks. Every item into the ZFW that leaves the gate will arrive with you at your destination.

For the sake of this scenario, with everything loaded on board, now comes the the Jet A. The weight of all that gas you'll need for your trip is made up of three elements. The first is your reserve fuel, enough for 30 minutes of cruise during the day or 45 minutes at night, which when added to your ZFW will give you your Landing Weight (LW).

The distance you'll fly and the fuel you'll need to fly there directly is called your Trip Fuel and is the largest portion of the fuel weight. As noted early, when the aircraft begins its take-off roll it is at its Take-off Weight (TOW), but when it pushes back from the gate it may exceed the MTOW, because the Maximum Design Taxi Weight (MTW) is higher than the MTOW. That's because your pilots have requested extra Taxi Fuel that will be burned while moving the aircraft from the gate to the take-off position.

Returning to the idea of the aircraft as a vessel that cannot be overfilled by the sum of the different elements. In a perfectly efficient use of the aircraft, the aircraft would depart the gate at MTW, leave the runway at its MTOW, and land at its LW just before using any reserve fuel. In an operational setting, Trip Fuel and Traffic Load are the two variable elements. On a short flight, less Trip Fuel is required, allowing the carrying of additional revenue cargo for example, along with the full load of passengers and their baggage.

Chart Credit Airbus

Singapore Airlines Boeing 777-300ER 9V-SWT

Now what does this mean for aircraft performance?

On a long flight, this is more of a balance with a larger weight of Trip Fuel required. On an extremely long flight, aircraft may even be restricted by how much payload they it can carry while reaching the aircraft's MTOW. Commercial jetliners, are rarely, if ever, restricted by their fuel volume, meaning aircraft range is a function of MTOW. And as commenter CM put it recently: 'Is the structure capable of supporting the passengers and payload, plus enough fuel to fly this mission, and does the wing produce enough lift to get it off the ground?'

This allows Emirates, for example, to fly from Dubai to Los Angeles on a 777-300ER, but with a 20 fewer seats on the 7,200nm mission based on the amount of Trip Fuel the airline needs to complete the mission while not exceeding its MTOW. Emirates still flies the mission because its 354 to 364-seat -300ER holds more passengers than the 266-seat 777-200LR that can fly the mission without restriction.

Now that we understand the balance between the Trip Fuel and Traffic Load, back at the manufacturer, ensuring the lowest possible MEW allows airlines to balance the fuel and load equation with more flexibility to fly their missions. An overweight aircraft eats directly into the revenue and/or mission capability of the airline.

The aircraft will use need use more thrust (and fuel) to move the heavier airframe, just as you have to press the accelerator pedal harder going up a hill when your car is fully loaded than when it's empty.

How far an aircraft can fly is also limited by aircraft thrust, runway length and condition and flap setting. And this is where all the debate over the A350-1000 not having enough thrust originates. With the aircraft set to receive a 5,000lb boost in thrust to its Trent XWB engines, Airbus is counting on the 350-seat jet accelerating to takeoff speed faster with more payload on the same runway.

An airframer can improve aircraft performance in many different ways and include, but are not limited to, the following. An increase in the MTOW can be achieved, for example, with a series of structural doublers that change the exterior of the aircraft, this wouldn't decrease the MEW, but does allow for the structure to carry more, allowing more Trip Fuel and/or Traffic Load flexibility to the operator.

For example, the newest A330-300 weight variant, WV54, increases the MTOW by 4,400lb (2,000kg) by changing the rivets on the lower fuselage shell but has a 230-300ft longer takeoff distance as the engine thrust is unchanged. The increase means the jet can carry 8,800lb (4t) more fuel for a 135nm longer range, but this comes from a 4,400lb (2t) reduction in the maximum ZFW in the form of payload capability.

More extensive structural changes to the aircraft are also possible, including wing and landing gear strengthening or even adding additional fuel tanks to the horizontal tail of the aircraft - like the 747-8I - or belly auxiliary fuel tanks. The result of these changes can produce distinctly different products like extended or longer range (ER or LR) models like those on the 777 or Airbus ACJs and Boeing BBJs configured for long-range missions.

At the end of the day, there are a myriad of factors that go into determining an aircraft's payload range capability in comparison to its maximum take-off weight. Overall,'aircraft design', in the words of one industry official is 'one big compromise'. Nothing comes free in the balancing act of designing flying machines.

1 comment:

aerospace hardware said...

The engineering that Airbus employs on most of their aircraft really gives Boeing some serious beating. So far in the last what, 5 years, the former can only respond with just the 767.

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